The background of the invention will be set forth in two parts:
1. Field of the Invention
This invention relates to radio frequency filter devices and more particularly to surface acoustic wave devices and charge coupled devices in tapped delay line configurations.
2. Description of the Prior Art
The tapped delay line (TDL) is well known as an important device for analog signal processing. With the addition of adjustable tap weights, it becomes a programmable transversal filter. Using a straight-forward synthesis procedure, the tap-weights can be selected so as to implement a wide range of transversal filter transfer functions. In general; however, the design range of the transversal filter is limited by the performance of the TDL and of the tap weight circuits.
It is fundamental to the design of all TDL transversal filters that the maximum bandwidth of the synthesized filter is the inverse of the intertap delay. Furthermore, the TDL frequency resolution capability is proportional to the overall delay time as described, for example, by H. E. Kallman in "Transversal Filters", Proceedings of the I. R. E. 28 (July 1940) pp. 302-310. Therefore, it can be seen that obtaining broad bandwidth and high resolution requires a long TDL with many closely spaced taps. Also, for proper transversal filter operation, it is important that the taps have low cross-talk. In the past, surface acoustic wave (SAW) TDL's with a large number of taps have been implemented, but the results were mostly not satisfactory because of propagation loss, tap reflections, tap to tap feed-thru and the interface problem of connecting the individual taps to the tap-weight circuits. In many ways the system requirements drive the TDL parameters to the limits of available technology.
Moving in another direction, the prospect of implementing a TDL using CCD (charge coupled devices) shift register technology has been considered. The function of the CCD TDL is essentially the same as the SAW device although the mechanism of delay is slightly different. First, the input to the CCD is an analog replica of the input signal, sampled at discrete times that are determined by a clock signal. The input sample (charge packet) is then transferred along the delay gates by sequential changes in the gate polarity. Under the influence of a primary clock, the surface potential is lowered ahead of the packet, and then the potential of the packet is raised causing charge to flow along the delay structure. Because the principal driving force causing the transfer between gates is a relatively slow diffusion process, the efficiency of charge transfer is generally strongly dependent on the clock speed. When insufficient transfer time is available, as at high clock rates, incomplete transfer results. This effect is referred to as charge transfer loss. Thesefore, it has generally been concluded that while TDL implementations utilizing CCD technology will provide good results in the frequency range of several MHz, the prospects of substantially extending bandwidth and delay are considered dim. In view of the limitations of these prior art techniques, it should be evident that a new technique involving a novel combination of SAW and CCD technologies which provides greater than 10 MHz bandwidth, several hundred independent taps, large dynamic range, programmability, small size and low power, would constitute a significant advancement of the art.